More than 700,000 foreigners who were supposed to leave the United States during a recent 12-month period overstayed their visas, the Homeland Security Department said Tuesday.

President Donald Trump has focused border security efforts on erecting a multibillion-dollar wall with Mexico. But the latest annual figures underscore how visa overstays are a big driver of illegal immigration. An estimated 40 percent of the roughly 11 million people in the country illegally stayed past their visas.

There were 701,900 visa overstays from October 2016 through September 2017 among visitors who arrived by plane or ship - more than the population of Vermont or Wyoming.

The total number of overstays is much larger but has not been quantified because it doesn't include how many people arrive by land.

The cost and technological hurdles to develop a checkout system at congested land crossings are enormous. Last year, authorities occasionally captured fingerprints from people in vehicles at three crossings with Mexico and plan to test facial recognition technology on pedestrians at two Arizona crossings with Mexico.

In 2016, Homeland Security published the number of overstays for the first time in at least two decades. From October 2015 through September 2016, there were 739,478 overstays among visitors who arrived by plane or ship.

Overstays accounted for 1.3 percent of the 52.7 million visitors who arrived by plane or ship during the latest period, an improvement from the overstay rate of 1.5 percent a year earlier.

Canada again occupied the top slot for overstays, followed by Mexico, Venezuela, the United Kingdom and Colombia. Nigeria, China, France, Spain and Germany rounded out the top 10.

The overstay rate was much higher among students and foreign exchange visitors, with 4.2 percent staying after their visas expired, a decline from 5.5 percent the previous year.